1. Field of the Invention
The present invention relates to a telephoto lens system for use in a single-lens reflex (SLR) camera.
2. Description of the Related Art
In a recent exchangeable lens system for a SLR camera, a high-speed focusing capability is required in order to cope with the automatic focusing system thereof. In order to provide a high-speed focusing capability, the weight of the focusing lens group needs to be lighter.
In a lens system in which all the lens groups are advanced for focusing, the weight of the lens groups to be moved becomes heavier; and accordingly, such a lens system is not suitable for a higher-speed focusing capability.
In known telephoto lens systems, many lens systems employing an internal-focusing system which is arranged to move only a lens group provided in the middle of the telephoto lens system, have been proposed.
For example, in Japanese Unexamined Patent Publication (JUPP) No. H08-327896, four lens elements constitute an internal-focusing lens group; and in JUPP No. H08-5907 and JUPP No. H06-201989, three lens elements constitute an internal-focusing lens group.
Furthermore, in JUPP No. H01-211711 and JUPP No. H06-138385, two lens elements constitute an internal-focusing lens group.
Here note that in JUPP No. H01-211711, a first lens group includes two positive lens elements made of a special low-dispersion glass (Abbe number of over 80) in order to correct aberrations, especially chromatic aberration; and in JUPP No. H06-138385, the rearmost (final) lens group includes a positive lens element and a negative lens element which are cemented to each other in order to correct distortion and spherical aberration.
As disclosed in JUPP No. H08-327896 and JUPP No. H08-5907, the internal-focusing lens group includes three or four lens elements, and the weight of the focusing lens group is lighter compared to that of a lens system in which all the lens groups are advanced for focusing. However, such an internal-focusing lens group is heavier than the internal-focusing lens groups disclosed in JUPP No. H01-211711 and JUPP No. H06-138385, and is not advantageous for high-speed focusing.
Furthermore, in JUPP No. H08-327896, JUPP No. H08-5907 and JUPP No. H06-201989, the distance between the focusing lens group and a lens group therebehind is very short, so that a range in which the focusing lens group is allowed to move is very small. Consequently, the minimum photographic distance becomes longer.
In JUPP No. H01-211711 and JUPP No. H06-138385, focusing is performed with two lens elements, so that a high-speed focusing operation is possible.
In JUPP No. H01-211711, the first lens group includes the two positive lens elements made of a special low-dispersion glass (Abbe number of over 80) in order to correct aberrations, especially chromatic aberration. The first lens group of a telephoto lens system is generally known to have an extremely large aperture; and moreover, if the two positive lens elements are made of a special low-dispersion glass (Abbe number of over 80), such an optical arrangement inevitably causes an increase in manufacturing costs.
In JUPP No. H06-138385, the rearmost (final) lens group includes a positive lens element and a negative lens element which are cemented to each other, because it has been understood that a single meniscus lens element cannot correct both distortion and spherical aberration. The cemented lens elements, however, generally increase manufacturing costs, compared with the single (meniscus) lens element.